[InstSimplify] Simplify saturating add/sub + icmp

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[InstSimplify] Simplify saturating add/sub + icmp
If a saturating add/sub has one constant operand, then we can
determine the possible range of outputs it can produce, and simplify
an icmp comparison based on that.
The implementation is based on a similar existing mechanism for
simplifying binary operator + icmps.
Differential Revision: https://reviews.llvm.org/D55735
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@349369 91177308-0d34-0410-b5e6-96231b3b80d8